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The Pons WED.26/4 Lab neuro # 8 The Pons is anterior to the cerebellum and connects the medulla oblongata to the midbrain , it is about 1 inch long. Anterior surface : Inside the pons we have the middle cerebellar peduncle which connects the cerebellum with pons. There is a shallow groove in the midline of anterior surface , the basilar groove, which lodges the basilar artery. On the anterolateral surface of the pons , the trigeminal nerve emerges on each side Each nerve consist of a smaller ,medial part ,known as the motor root, and a larger, lateral part, known as the sensory root. In the groove between the pons and the medulla ablongata there emerge, from medial to lateral, the abducent , facial, and vestibulocochlear nerve. These three nerves are at the junction between medulla and pons Note : multiple cranial nerves come out from pons and as we know each cranial nerve have anterior and posterior roots . ***{while studying this sheet you must see the pics in the book & sildes} 1 Posterior surface: The posterior surface is limited laterally by sup cerebellar peduncles and is divided into symmetrical halves by a median sulcus. Lateral to this sulcus is the medial eminence, which is bounded laterally by a sulcus the sulcus limitans. The inferior end of the medial eminence is slightly expanded to form the facial colliculus. Internal structure of the pons: * the pons is commonly divided into a posterior part, the tegmentum (mostly contains nuclei), and an anterior basal part (mostly contains fibers) * the structure of the pons may be studied at two levels : (1) level of facial nucleus (lower pons) (2) level of trigeminal nuclei (upper pons) * trapezoid fibers separate the tegementum from the basal part * Level of lower pons (1) Tegmentum: Medial lemniscus is situated in the most anterior part of the tegmentum. At this level we can see: - The 4th ventricle - Superior peduncle (more prominent at higher level) which connects the cerebellum with midbrain. - Part of the inferior peduncle - Level of facial folliculus which consists of 2 motor nuclei (nucleus of abducent nerve & nucleus of facial nerve) 2 Facial nucleus lies posterior to the lateral part of the medial lemniscus. The fibers of facial nerve wind around the nucleus of the abducent nerve, producing the facial colliculus. The fibers of the facial nerve then pass anteriorly between the facial nucleus and the superior end of the nucleus of the spinal tract of the trigeminal nerve. NOTE: Facial nerve lies between medial lemniscus & nucleus of the spinal tract of the trigeminal nerve (which is the extension of Substantia gelatinosa which extend ton end at the lower pons). - medial longitudinal fasciculus (MLF) : Contains ascending and descending fibers near the midline. It is the main pathway that connects vestibular and cochlear nuclei controlling the extraocular muscles (occulomotor, trochlear, and abducent nuclei) for the movement of head, neck, and eyes. - medial vestibular nucleus: Situated lateral to the abducent nerve and in close relationship to the inferior cerebellar peduncle. - trapezoid: Is made up of fibers derived from cochlear nuclei and the nuclei of the trapezoid body. NOTE: Post view Vestibular and cochlear nuclei are both sensory nuclei Vestibular complex is made of 4 nuclei ( sup+ inf+ med+ lat) in each section we can see part of these nuclei or none it depends on the section Ex: in the lower section we can see med + inf The ascending part of the MLF connects vestibular and cochlear nuclei Medial longitudinal fasciculus (ascending MLF) Medial vestibulospinal tract (descending MLF) Only remainents of cochlear nuclei can be seen in lower Pons. (2) Basilar part (Mostly containing fibers ) It contains small masses of nerve cells called Pontine nuclei 3 The axons of these cells give origin to the transverse fibers (transverse pontine fibers) of the pons which crosses the midline and intersects the corticospinal and corticonuclear tracts breaking them into small bundles. This connection forms the main pathway linking the cerebral cortex to the cerebellum: * Corticospinal tracts doesn’t form pyramids like in the medulla because it doesn’t project to the surface it only passes inside. * Corticonuclear tracts the same as the corticospinal but it reaches the cranial nerves. * abducent nerve abduction of the eye Facial nerve muscles of the face * Level of upper pons Same as the lower pons have : -anteriorly basal part -posteriorly tegmentum Always post to medial lemniscus spinal lemniscus is present spinal lemniscus have part of the spinothalamic tract and the other part tectothalamic tract. * The Superior peduncle is larger in this section than in the lower pons but not larger than in the midbrain Inferior peduncle is not present here, only in medulla oblongata & pons. Middle peduncle is present in basal part and more prominent because it goes to the pons & cerebellum *what makes this level special is the presence of two trigeminal nuclei: (1) Motor trigeminal nucleus Medially, smaller, within reticular formation (2) Sensory trigeminal nucleus Laterally, larger, extension to the spinal trigeminal nucleus These give rise to the 5th cranial nerve (trigeminal nerve) which is a very important nerve. 4 * Pontine nuclei receives afferent fibers from cortex (temporal lobe, frontal lobe) called frontopontine and tempropontine fibers it relay and synapses there. Then it sends transverse pontine fibers (transverse pontocerebellar fibers) to the middle cerebellar peduncle then to the cerebrum. This is the main pathway that sends information from cerebrum to cerebellum. * The corticospinal tract passing through this area won’t bulge to the surface forming pyramids as we said why??? Because the transverse fibers while passing through this area it will cause the break down of corticospinal bundles preventing the aggregation of these fibers and so prevents forming the pyramids. GOOD LUCK ALL : D Done by: Razan Amayreh 5